With recent development of information communication technologies, in the field of intelligent transport systems (ITS), various researches and developments have been made for inter-vehicle communications by which information communication is performed by radio or the like among traveling or stopped vehicles.
The inter-vehicle communication is classified into point-to-point communication for communicating between vehicles in a close proximity of each other using a directional antenna, and point-to-multipoint or multipoint-to-multipoint communication for transmitting information regarding a driver' vehicle to other vehicles in a close proximity of the driver's vehicle using a non-directional antenna (for example, Kaichi Fujimura et al.; “A Collaborative MAC Protocol of Inter-Vehicle and Road to Vehicle Communications”, The second ITS symposium, Japan, ITS JAPAN, 6-8 Dec. 2003, pp. 361-366).
The communication using the directional antenna has been examined to be used for active systems for controlling and automatically steering vehicles using inter-vehicle distance control, platooning, or the like, and on the other hand, the communication using the non-directional antenna has been considered to be suitable for passive systems for providing information and alarming.
As one of the communication systems using the non-directional antenna for providing information and alarming, a human-centered ITS view aid system has been developed to prevent pileups by detecting statuses of a road, a vehicle, and a driver when the vehicle travels at a high speed, and providing, using the inter-vehicle communication, following vehicles and the like with an emergency status such as a collision, an icy road surface, or a low conscious level of the driver, by displaying the status in real time and clearly even for aged drivers (for example, Sadayuki Tsugawa; “Future of Inter-Vehicle Communications ITS View Aid”, Humatronics, Japan, Society of Automotive Engineers of Japan, Inc., 6 Feb. 2004, No. 06-04, pp. 38-43). As further examples, it is disclosed that experiments in transmitting acceleration/deceleration information, braking information, and the like from a preceding vehicle to a following vehicle have been conducted in a cooperative driving phase I project by Jouhou System Kougaku Laboratory (JSK) as well as in a CarTALK2000 project by the European Union (see the non-patent document by Sadayuki Tsugawa).
On the other hand, researches have been made for driver assistance systems for warning a driver about an approaching vehicle which the driver cannot see, by transmitting position information using the inter-vehicle communication in an intersection where a driver's view is obstructed.
Furthermore, a communication device for vehicles has been conventionally disclosed to have a function of detecting in real time traffic congestion information using the inter-vehicle communications (for example, Japanese Patent Laid-Open No. 2003-272095 publication).
Still further, a method has conventionally been disclosed for propagating data packets between vehicles using the inter-vehicle communications (for example, Japanese Patent Laid-Open No. 2002-123892 publication).